1. Field of the Invention
The present invention is related to ultrasonic diagnostic techniques and, more particularly, is directed towards an ultrasonic diagnostic technique for simplifying wall thickness measurement of cardiac structures.
2. Description of the Prior Art
Echocardiography constitutes a popular non-invasive technique in medical diagnostics which enables the visualization and measurement of cardiac structures. The technique comprises transmitting an ultrasonic pulse or signal towards the cardiac structure of interest and detecting the received echo signal reflected therefrom. A multi-surfaced cardiac structure will be responsible for a plurality of echo signals reflected back to the receiver. Detection of the depth from which the multiple signals originate at a particular instant of time provides data whereby the distance between two surfaces or components may be ascertained.
The pulse-echo ultrasonic technique described above has been particularly valuable in the quantification of ventricular septal and posterior wall thicknesses. See for example the Henry et al article "Asymmetric Septal Hypertrophy (Ash): Echo-Cardiographic Identification of the Pathogonomonic Anatomic Abnormality of IHSS" in the February, 1973, issue of Circulation, Volume 47 at page 677. The posterior wall of the left ventricle comprises an inwardly facing endocardial surface and an outer epicardial surface. The epicardial surface lies adjacent a wall of the lung, and the area is frequently referred to as the epicardial-lung interface.
In echocardiographic analysis of the left ventricular structure, it has been found that the signal reflected from the epicardial-lung interface is much stronger than that from the surrounding myocardium and lung. This is primarily due to the presence of air in the lung which causes a large change in the acoustic impedance of the ultrasonic signal at the epicardial-lung interface. The relatively large magnitude of the signal reflected from the epicardial-lung interface is often obliterated when the gain of the receiver is increased sufficiently to record the endocardial signal. In other words, as a result of the limited dynamic range and grey scale available in the typical displays or direct-writing recorders, effective simultaneous viewing of the epicardial and endocardial wall surfaces is difficult.
A swept gain feature is available on standard ultrasonic equipment and is utilized to normalize signal strength from various depths, thereby reducing the need for grey scale. However, the distance from the ultrasonic transducer to the myocardium changes both during the cardiac cycle and in accordance with the positioning of the transducer. Thus, the swept gain feature alone is not significantly helpful in clarifying the epicardial and endocardial surfaces.
Another attempted solution has been to first manually set the receiver to record several heartbeats at a low receiver gain to emphasize the epicardium, and then manually increase the gain and sensitivity to emphasize the endocardium surface. Data from the two sections of echocardiogram may then be combined to yield wall thickness. Although this approach has provided important diagnostic information, it requires stable transducer placement during both measurments which often is quite difficult to achieve. Further, it assumes that the heart dimensions do not change significantly with respiration, an assumption which can lead to erroneous readings. Further, this technique, being manual in nature, is quite tedious and time consuming.
The echo signal received in a typical diagnostic pulse-echo system has a useful dynamic range on the order of 100 dB. Approximately 50 dB of this range results from attenuation of echos from the deeper structures and therefore can be removed with swept gain. This leaves approximately 50 dB for display and recording. Most oscillographic recording systems, however, are unable to retain the 50 dB range. For example, a brightness modulated cathode ray tube is capable of only about 20 dB, while the popular direct-print paper retains only 15 dB (four grey levels). A variety of compression techniques may be utilized to reduce the signal's range, but resulting images are somewhat less than satisfactory.